CODEWITHSUNDEEP
c#
Varun Kumar
Varun Kumar

Reputation: 478

Picture box always throws object reference not set to instance of object error

I have a class which holds a picturebox. I have created an array of objects of my class, but when I try to create a new picture box, I am getting an error stating, object reference is not set to an instance of object.

Here is how I am creating pictureboxes

 GreyHound[] dog;

    public Form1()
    {
       InitializeComponent();
       dog[1].MyPictureBox = new PictureBox();
       dog[1].MyPictureBox = pictureBox1;

       dog[2].MyPictureBox = new PictureBox();
       dog[2].MyPictureBox = pictureBox2;

       dog[3].MyPictureBox = new PictureBox();
       dog[3].MyPictureBox = pictureBox3;

       dog[4].MyPictureBox = new PictureBox();
       dog[4].MyPictureBox = pictureBox4;

Edited Code :

GreyHound[] dog;

    public Form1()
    {

       InitializeComponent();
       dog = new GreyHound[4];
       dog[0].MyPictureBox = new PictureBox();
       dog[0].MyPictureBox = pictureBox1;

       dog[1].MyPictureBox = new PictureBox();
       dog[1].MyPictureBox = pictureBox2;

       dog[2].MyPictureBox = new PictureBox();
       dog[2].MyPictureBox = pictureBox3;

       dog[3].MyPictureBox = new PictureBox();
       dog[3].MyPictureBox = pictureBox4;



    }

Upvotes: 0

Views: 2222

Answers (2)

Daniel Hilgarth
Daniel Hilgarth

Reputation: 174289

The code you have has several severe problems:

  1. The reason for your NullReferenceException is two-fold:
    1. You declare an array of GreyHound, but you never initialize it. That means, that dog is null and trying to access dog[1] results in a NullReferenceException.
    2. If you would fix that, you would still have another problem:
      Assuming that GreyHound is a reference type that means that you will have to create an instance for each item in the array. With your current code, all items in the array are null which leads to a NullReferenceException when trying to access the property MyPictureBox on any item in the array, because dog[1] would be null.
  2. For every item in the array, you first assign a new instance of a PictureBox to the property MyPictureBox but you overwrite that value in the very next line with pictureBoxX.
  3. As we are in C#, arrays are zero indexed, so the first item has the index 0 and not 1.

Fix your code like so:

GreyHound[] dog = new GreyHound[4]; // solves problem 1.1

dog[0] = new GreyHound(); // solves problem 1.2
dog[0].MyPictureBox = pictureBox1;

dog[1] = new GreyHound(); // solves problem 1.2
dog[1].MyPictureBox = pictureBox2;

dog[2] = new GreyHound(); // solves problem 1.2
dog[2].MyPictureBox = pictureBox3;

dog[3] = new GreyHound(); // solves problem 1.2
dog[3].MyPictureBox = pictureBox4;

Upvotes: 3

gzaxx
gzaxx

Reputation: 17590

You do not initialize you array anywhere:

dog = new GreyHound[4];

also array indexing starts from 0 so:

dog[0].MyPictureBox = new PictureBox();
dog[0].MyPictureBox = pictureBox1;

dog[1].MyPictureBox = new PictureBox();
dog[1].MyPictureBox = pictureBox2;

dog[2].MyPictureBox = new PictureBox();
dog[2].MyPictureBox = pictureBox3;

dog[3].MyPictureBox = new PictureBox();
dog[3].MyPictureBox = pictureBox4;

Upvotes: 2

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